The present invention relates to a liquid developer used for development of electrostatic latent images in electrophotography and electrostatic recording and more particularly it relates to a liquid developer which has improved storage life, stability and fixability and is prevented from sticking of the toner to inside and outside of the developing apparatus during long-term using and thus is improved in its maintenance.
A large number of liquid developers for electrostatic photography have already been known. The most popular are those which are prepared by mechanically dispersing color pigments or dyes such as carbon black, cyanine blue, nigrosine and oil dyes in highly insulating media together with resins such as rosins, alkyd resins, acrylic resins and synthetic rubbers by a ball mill, an attritor, a homogenizer or the like and adding thereto metallic soaps, amines, higher fatty acids or the like to give an electric charge to the dispersed particles of the dispersion.
However, the liquid developers obtained by the above process suffer from the problems that particle size distribution of the dispersed particles per se is broad and precipitates are produced due to sedimentation of coarse particles and that dispersion stability of the dispersed particles is not enough and agglomeration and precipitation of the particles occur with elapse of time and so, stable images can be obtained with difficulty in long-term use.
As mentioned in Japanese Patent Kokoku Nos. 53-54029 and 57-12985, there are processes for preparation of liquid developers according to which a polymerizable vinyl group is previously introduced into a precursor polymer by a polymer reaction and a monomer is polymerized in the presence of the said precursor polymer to produce a graft copolymer which stabilizes the dispersed polymers thereby to realize polymer particles superior in dispersion stability and the resulting polymer particles are colored with dyes.
These processes have problems in control of introduction of the vinyl group and in reproducibility and formation of gel is often brought about or sometimes the dispersed particles cannot be produced stably. Thus, there occurs a problem when dispersed particles uniform in particle size distribution are to be stably produced.
Furthermore, Japanese Patent Kokai (Laid-Open) Nos. 59-86174, 59-177572, 59-212850, 59-212851, 60-164757, 60-179751, 60-185962, 60-185963, 60-252367, 61-116364 and 61-116365 disclose that in the presence of a polymer soluble in a highly insulating medium, a monomer which is soluble in said medium, but becomes insoluble when polymerized is polymerized and the resulting resin dispersion is used as a liquid developer.
Although the liquid toners obtained by the above processes are superior in stability in industrial production and dispersion stability, when they are actually used as liquid toners, redispersibility of the dispersed particles in the medium is not enough and the dispersed particles after evaporation of the medium are in the form of a film and cannot be readily removed and as a result, the film sticks to inside and outside of the developing apparatus. Therefore, the apparatus per se is considerably contaminated and especially, the toner film which stick to the rolls in the developing part causes insufficient squeezing by the rolls. Accordingly, maintenance of the developing apparatus is always needed to inhibit generation of these troubles.